For the treatment of macular degeneration or retina diseases such as for example diabetic retinopathy, intravitreal injections of drugs have proven successful. In intravitreal injections a drug formulation is directly injected into the vitreous humor (VH), that is, into the clear gel filling the space between lens and retina of the eyeball of for example humans. This is not least due to the fact that in intravitreal injections the fraction of an administered dose of unchanged drug that reaches the retina is high, thus intravitreal injections usually having high bioavailability.
However, in-vitro tests to study for example stability of a drug formulation in vitreous humor (VH) have proven little useful when simulating real in vivo conditions. VH, when not in its natural environment, degenerates fast and a pH value of VH may increase rapidly due to accumulation of degenerated products in the VH. Thus, tests may not represent the actual situation in an eye of a living person, especially not over long periods of time, such as for example several days. In more recent test systems, the physiological pH value of VH may be stabilized by applying a buffering system. Therein, degradation products are allowed to leave the VH though a semi-permeable membrane into a buffer solution.
However, these systems do not allow to simulate different barrier conditions for drug formulations. Especially, they do not allow to simulate different barrier conditions as for example provided by a posterior segment tissue of an eye.
Thus, there is need for an improved in vitro method and apparatus for simulating physiological environment to analyse the behaviour of substances such as for example macromolecules. Especially, there is need for an in vitro method and apparatus for analyzing long term stability of substances in different simulated physiological environments.